1. Field of the Invention
The present invention relates to a numerical controller for controlling a machine tool, and more particularly, to a numerical controller for controlling a feed speed for linear or circular interpolation.
2. Description of the Related Art
In a numerical controller for controlling a machine tool, speed control is performed for an arbitrarily assigned feed speed. In assigning a high feed speed, if linear or circular interpolation with a block length that allows a movement to be completed within an interpolation period is commanded with multi-block overlap enabled, a shape that is formed by connecting control points for each interpolation period as a unit connected by straight lines is machined. In some cases, therefore, programmed shapes in intermediate blocks may fail to be reflected.
Thus, if a programmed shape is expected to be maintained during machining, the speed is conventionally controlled so that the feed speed between blocks is reduced to zero by an exact stop, or the shape is maintained by means of a deceleration function to suppress speed or acceleration change attributable to the change of movement direction.
If a high tolerance is set for the deceleration function based on the speed or acceleration change, movement may fail to be completed in an interpolation period. In such a case, the speed cannot be reduced to a proper level, so that the programmed shape cannot be maintained. Although the speed must be further reduced to prevent this, there may be a request to avoid speed reduction in an undesired place for high-speed machining, or in contrast, the programmed shape may be expected to be maintained. To maintain the programmed shape, an exact stop command or feed speed must be changed in programming in consideration of the feed speed and block length in the stage of programming. In many cases, however, the feed speed is kept constant during machining, so that it is not easy to change programs in consideration of the feed speed and block length.
Japanese Patent Application Laid-Open No. 6-95727 discloses a technique in which a critical speed obtained from the time required for preprocessing means to read and analyze each block of a machining program is compared with a feed speed, in a feed speed clamping system of a numerical controller, and the lower of these speeds is clamped and output as a commanded speed.
According to the technique described above, however, the critical speed is calculated based on the time required to read and analyze each block of the machining program, so that an appropriate speed sometimes cannot be determined if the blocks vary in shape.
On the other hand, Japanese Patent Application Laid-Open No. 5-143145 discloses a technique in which an initial value of machining block processing time is previously set, and the feed speed is controlled according to functional patterns of read blocks of an NC program.
According to the technique described above, however, the allowable speed is calculated by analyzing the functional patterns of the machining blocks, so that the calculated speed may possibly vary depending on the time required for the analysis.